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Dependency resolution - Understanding Operators | Operators | OpenShift Container Platform 4.5
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Operator Lifecycle Manager dependency resolution

This guide outlines dependency resolution and custom resource definition (CRD) upgrade lifecycles with Operator Lifecycle Manager (OLM) in OpenShift Container Platform.

About dependency resolution

OLM manages the dependency resolution and upgrade lifecycle of running Operators. In many ways, the problems OLM faces are similar to other operating system package managers like yum and rpm.

However, there is one constraint that similar systems do not generally have that OLM does: because Operators are always running, OLM attempts to ensure that you are never left with a set of Operators that do not work with each other.

This means that OLM must never:

  • install a set of Operators that require APIs that cannot be provided, or

  • update an Operator in a way that breaks another that depends upon it.

CRD upgrades

OLM upgrades a custom resource definition (CRD) immediately if it is owned by a singular cluster service version (CSV). If a CRD is owned by multiple CSVs, then the CRD is upgraded when it has satisfied all of the following backward compatible conditions:

  • All existing serving versions in the current CRD are present in the new CRD.

  • All existing instances, or custom resources, that are associated with the serving versions of the CRD are valid when validated against the validation schema of the new CRD.

Adding a new CRD version

Procedure

To add a new version of a CRD:

  1. Add a new entry in the CRD resource under the versions section.

    For example, if the current CRD has a version v1alpha1 and you want to add a new version v1beta1 and mark it as the new storage version, add a new entry for v1beta1:

    versions:
  - name: v1alpha1
    served: true
    storage: false
  - name: v1beta1 (1)
    served: true
    storage: true
    1 New entry.

  2. Ensure the referencing version of the CRD in the owned section of your CSV is updated if the CSV intends to use the new version:

    customresourcedefinitions:
  owned:
  - name: cluster.example.com
    version: v1beta1 (1)
    kind: cluster
    displayName: Cluster
    1 Update the version.

  3. Push the updated CRD and CSV to your bundle.

Deprecating or removing a CRD version

Operator Lifecycle Manager (OLM) does not allow a serving version of a custom resource definition (CRD) to be removed right away. Instead, a deprecated version of the CRD must be first disabled by setting the served field in the CRD to false. Then, the non-serving version can be removed on the subsequent CRD upgrade.

Procedure

To deprecate and remove a specific version of a CRD:

  1. Mark the deprecated version as non-serving to indicate this version is no longer in use and may be removed in a subsequent upgrade. For example:

    versions:
  - name: v1alpha1
    served: false (1)
    storage: true
    1 Set to false.

  2. Switch the storage version to a serving version if the version to be deprecated is currently the storage version. For example:

    versions:
  - name: v1alpha1
    served: false
    storage: false (1)
  - name: v1beta1
    served: true
    storage: true (1)
    1 Update the storage fields accordingly.

    In order to remove a specific version that is or was the storage version from a CRD, that version must be removed from the storedVersion in the status of the CRD. OLM will attempt to do this for you if it detects a stored version no longer exists in the new CRD.

  3. upgrade the CRD with the above changes.

  4. In subsequent upgrade cycles, the non-serving version can be removed completely from the CRD. For example:

    versions:
  - name: v1beta1
    served: true
    storage: true

  5. Ensure the referencing CRD version in the owned section of your CSV is updated accordingly if that version is removed from the CRD.

Example dependency resolution scenarios

In the following examples, a provider is an Operator which "owns" a CRD or API service.

Example: Deprecating dependent APIs

A and B are APIs (CRDs):

  • The provider of A depends on B.

  • The provider of B has a subscription.

  • The provider of B updates to provide C but deprecates B.

This results in:

  • B no longer has a provider.

  • A no longer works.

This is a case OLM prevents with its upgrade strategy.

Example: Version deadlock

A and B are APIs:

  • The provider of A requires B.

  • The provider of B requires A.

  • The provider of A updates to (provide A2, require B2) and deprecate A.

  • The provider of B updates to (provide B2, require A2) and deprecate B.

If OLM attempts to update A without simultaneously updating B, or vice-versa, it is unable to progress to new versions of the Operators, even though a new compatible set can be found.

This is another case OLM prevents with its upgrade strategy.